In recent years, lithium ion batteries are installed in mobile devices, such as a digital camera and a cellular phone, as a secondary battery. Generally, lithium ion batteries are prone to overcharge, overdischarge and overcurrent, and a lithium ion battery is arranged in a form of a battery pack containing a protection circuit which detects overcharge, overcurrent and overdischarge of the battery to protect the battery from being damaged.
In a battery pack, a secondary battery monitoring circuit may be contained. This secondary battery monitoring circuit includes a temperature sensor which is arranged to detect a voltage change, corresponding to a temperature change in the battery pack, to determine a state of a lithium ion battery, such as a battery residual quantity. In this case, an additional communication terminal for transmitting an output signal from the secondary battery monitoring circuit to a mobile device is disposed in the battery pack, and the secondary battery monitoring circuit receives the signal indicating the state of the battery pack output from this communication terminal, and manages the operating state of the battery pack.
Conventionally, a charging device which receives a control signal for controlling charging of a battery pack from a communication terminal disposed in the battery pack is known. For example, refer to Patent Document 1 listed below.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2000-209788